Add GemFire Support

Fixes GH PR #148 and PR #308 implementing a GemFire Adapter to support
clustered HttpSessions in Spring Session.

* Resolve SGF-373 - Implement a Spring Session Adapter for GemFire backing
a HttpSession similar to the Redis support.
* Add Spring Session annotation to enable GemFire support with
@EnableGemFireHttpSession.
* Add extesion of SpringHttpSessionConfiguration to configure GemFire using
GemFireHttpSessionConfiguration.
* Add implementation of SessionRepository to access clustered, replicated
HttpSession state in GemFire with GemFireOperationsSessionRepository.
* Utilize GemFire Data Serialization framework to both replicate
HttpSession state information as well as handle deltas.
* Utilize GemFire OQL query to lookup arbitrary Session attributes by name,
and in particular the user authenticated principal name.
* Implment unit and integration tests, and in particular, tests for both
peer-to-peer (p2p) and client/server topologies.
* Set initial Spring Data GemFire version to 1.7.2.RELEASE, which depends
on Pivotal GemFire 8.1.0.
* Add documentation, Javadoc and samples along with additional Integration
Tests.

Fixes gh-148

samples/httpsession-gemfire-clientserver/src/main/java/sample/Initializer.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2002-2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package sample;
+
+import org.springframework.session.web.context.AbstractHttpSessionApplicationInitializer;
+
+// tag::class[]
+public class Initializer
+		extends AbstractHttpSessionApplicationInitializer { // <1>
+
+	public Initializer() {
+		super(ClientConfig.class); // <2>
+	}
+}
+// end::class[]